1. Field of the Invention
The present invention relates to an image printing apparatus which prints a one-line image in the main scanning direction in accordance with image data with reference to a dot clock as the basis of each pixel forming an image, and also prints a one-page image by repeating in the sub-scanning direction one-line image printing performed in the main scanning direction.
2. Description of the Prior Art
As an image printing apparatus, an apparatus for printing a one-line image in the main scanning direction in accordance with image data and printing a one-page image by repeating in the sub-scanning direction one-line image printing performed in the main scanning direction is known.
Image printing in the main scanning direction in accordance with image data is performed with reference to a clock signal (to be referred to as a “dot clock” hereinafter in this specification) as the basis of each pixel to be formed.
For example, in an electrophotographic image printing apparatus, a laser beam modulated in accordance with image data is scanned in the main scanning direction, and an image is concurrently formed on an image carrier rotating in the sub-scanning direction with the laser beam.
In this case, a laser beam is modulated with image data with reference to a clock signal (pixel clock) called a dot clock.
The arrangement of a writing unit as the main component of an image printing apparatus using a laser beam in this manner will be described with reference to FIG. 14.
In a writing unit circuit section 200, a laser diode (LD) 260 emits a laser beam LB on the basis of a laser driving signal LS generated on the basis of a dot clock.
The laser beam LB from the LD 260 passes through a collimator lens 261 and cylindrical lens 262 and is then scanned in the main scanning direction by a polygon mirror 263.
The laser beam LB scanned by the polygon mirror 263 is adjusted to be scanned at a constant velocity by an fè lens 264. The laser beam further passes through a cylindrical lens 265 to strike a photosensitive drum 10.
Sub-scanning operation is then performed by rotating the photosensitive drum 10 during this main-scanning operation. By repeating main-scanning operation in the sub-scanning direction, a one-page image is printed.
Note that part of the laser beam LB scanned by the polygon mirror is guided to an index sensor 266 to detect the timing.
In an image printing apparatus designed to print a one-page image by repeating such image printing operation, performed in the main scanning direction, in the sub-scanning direction, the main-scanning magnification (the magnification in the main scanning direction) may differ from a desired magnification due to the power characteristics and mount precision of various kinds of lens systems, the planarity and mount precision of various kinds of mirrors, the planarity and mount precision of a photosensitive drum as an image printing medium (image carrier), individual differences thereof, and the like. That is, an offset may occur in the main scanning direction.
An invention designed to change the frequency of a dot clock to adjust such a main-scanning magnification (eliminate an offset in the main scanning direction) is disclosed in, for example, patent reference 1 below.
A plurality of writing units may be provided for the above image printing apparatus to allow it to print a color image. In this case, if offsets in the main scanning direction occur between the respective writing units, a proper color image cannot be printed.
In order to eliminate such offsets in the main scanning direction, the frequency of a dot clock must be changed for each color. Conventional techniques of this type are disclosed in, for example, the following two patent references:
patent reference 1: Japanese Unexamined Patent Publication No. 2000-199868 (page 4; FIG. 7)
patent reference 2: Japanese Unexamined Patent Publication No. 2000-202648 (page 13; FIG. 1)
Repeated studies by the present inventors show that the following problems arise even though the main-scanning magnification is adjusted to coincide with the desired magnification by the techniques disclosed in the two patent references described above.
Even if the distance between the start and the end in the main scanning direction is made equal to a desired distance, a halfway point, e.g., an intermediate point between the start and the end, may not always coincide with a desired position.
In other words, in some case, although the two ends, i.e., the start and the end, coincide with desired positions, a slight offset (halfway offset) may have occurred between the two ends.
In the field of offset printing or the like, in particular, not only expansion/contraction between the start and the end but also a pixel offset at a halfway point (halfway offset) may pose serious problems.
The factors that cause such halfway offsets include the power characteristics and mount precision of various kinds of lens systems, the planarity and mount precision of various kinds of mirrors, the planarity and mount precision of a photosensitive drum as an image printing medium (image carrier), individual differences thereof, and the like. An enormous cost is required to manufacture and mount these components with high precision without causing any halfway offset.
In printing a color image, if such halfway offsets occur in different proportions among the respective colors, the offsets appear as noticeable color misregistration, posing a serious problem.
If the image printing apparatus is an image printing apparatus capable of printing images on the two surfaces of an image recording sheet, even if the distance between the start and the end in the main scanning direction is adjusted to a desired value, a user may recognize the difference between the distances on the two surfaces as an offset.
This is a case wherein an image recording sheet having images printed on its obverse and reverse surfaces is held up against light, an offset between the obverse and reverse surfaces is recognized. Obviously, in this case, a halfway offset may have occurred.